In one embodiment, the present disclosure addresses the need for improving the surface finish—reducing the surface roughness—of surfaces created on a work piece by a face milling tool. A face milling tool, also known as a face mill, has one or, more generally, multiple primary cutting teeth affixed to a face mill body around the face mill body circumference, generally substantially equally spaced, and aligned to one another in both the axial and radial dimensions of the face mill. Each primary cutting tooth is generally made up of a replaceable cutting insert and has provisions for attachment to the face mill body. A face mill is operated by attaching it to a spindle of a machine tool. The spindle rotates to produce a cutting motion at a relatively high cutting speed, also referred to herein as the tangential speed, while the machine provides a feeding motion of the work piece or the face mill, relative to the other, that occurs in the plane to which the spindle axis is perpendicular. The face mill removes a shallow layer of material from the work piece creating, with the tips of the primary cutting teeth, a new surface on the work piece that is substantially parallel to the plane of the feeding motion. Upon that surface and at a smaller, microscopic scale is the surface roughness. The surface roughness is comprised of a series of radiused feed grooves that trace the nearly-circular cutting motion of the primary cutting teeth. The feed grooves exhibit cusps/peaks occurring where adjacent feed grooves overlap one another leaving the radiused valleys of the feed grooves between the cusps/peaks.
The feeding action may be quantified as a distance traveled in the time it takes for one revolution of the face mill, referred to as the feed per revolution. Of greater significance as related to surface roughness is the feed per primary tooth, which is the feed per revolution divided by the number of primary cutting teeth affixed to the face mill cutter body. The feed per primary tooth dictates the distance between the microscopic peaks—the widths of the feed grooves.
Surface roughness may be characterized by one or more of numerous quantitative parameters, such as the roughness average value that is generally referred to as Ra. Ideally, the roughness average value is proportional to the square of the feed per primary tooth and inversely proportional to the radius on the tips of the primary cutting teeth. In practice the roughness average value will exhibit these types of proportional and inversely proportional trends; however, because the multiple primary cutting teeth are not perfectly aligned with one another, the roughness average value in practice will always be higher (a rougher surface) than the ideal value. While the radial misalignments of the primary cutting teeth have a deleterious effect on surface roughness by perturbing the widths of the feed grooves from their ideally equal widths, the axial misalignments of the primary cutting teeth have an even greater deleterious effect on surface roughness by also perturbing the depths of the feed grooves relative to their ideally equal depths.
Thus, four parameters combine to impact the surface roughness—feed per primary tooth, tool tip radius (often referred to as the corner radius, or sometimes the nose radius), tooth-to-tooth axial misalignments, and tooth-to-tooth radial misalignments. Assuming one has reduced the misalignments to be as small as possible by applying the degree of effort that can be afforded, it is the feed per primary tooth and the corner radius that are adjusted to achieve the desired/specified surface roughness on a face milled surface. Decreasing the feed per primary tooth, due to its squared effect on the average roughness (Ra), has the greater impact on decreasing/improving (making more smooth) the surface roughness, but, holding all other cutting conditions constant, such as spindle speed, this also results in a proportionate decrease in productivity. Increasing the corner radius will result in a proportionate decrease/improvement in surface roughness, but it also tends to direct a larger percentage of the cutting forces acting between the primary cutting teeth and the work piece into the axial direction, which can lead to structural deflections that result in dimensional error in the location of the face milled surface produced.
When producing a machined surface, it is common to take multiple passes, including one or more roughing passes to more rapidly remove larger amounts of material without concern for the aforementioned dimensional error or higher surface roughness, followed by a finish pass at a lower rate of material removal to facilitate meeting the dimensional and surface roughness requirements. To achieve particularly low surface roughness without excessively reducing the feed per primary tooth and, likewise, in the presence of some level of tooth-to-tooth misalignments that always exist in practice, one or more secondary “wiper” teeth may be added to the face mill. When viewed in the direction that is tangential to the face mill body (the cutting motion direction), wiper teeth have either a straight cutting edge or a cutting edge with a very large radius or “crown” that is much larger than the corner radius of the primary cutting teeth. Wiper teeth serve to remove the cusps/peaks of the surface roughness geometry. Because Ra is inversely proportional to the radius of the cutting edge, and the radius of the wiper is either very large (or infinite in the case of a non-radiused/straight wiper tooth cutting edge), the wiper can create a much smaller Ra value even if the feed per wiper tooth is larger than the feed per primary tooth. Of course, if there is more than one wiper tooth, the multiple wiper teeth must be carefully aligned in the axial direction, and for that reason, there are generally far fewer wiper teeth than primary cutting teeth, which is consistent with the ability to accommodate higher feed per wiper tooth than feed per primary tooth.
Wiper teeth, or rather the indexible cutting insert that makes up the cutting portion of the wiper tooth, are usually common-size square or rectangular cutting inserts made from one of the many cutting insert materials (e.g., tungsten carbide, ceramic, cubic boron nitride, etc., either with or without a coating) that are well known to those working in the field. Viewing in the direction of the axis of the face mill, the wiper tooth cutting edge is substantially straight and has finite length. Wiper teeth are set with their cutting edge length running substantially radially outward from the axis of the face mill. They are set at an axial position on the cutter body so the wiper cutting edge protrudes axially toward the machined surface just slightly more than the furthest protruding primary cutting tooth. The added protrusion of a wiper tooth may be up to approximately 0.003 inch (75 micron), sometimes less and sometimes more; it is desired to keep this added protrusion, or wiper depth, as small as possible while still assuring the wiper removes the entirety of all the cusps/peaks down to the lowest of the feed groove valleys. When a wiper has a non-radiused straight cutting edge, the wiper teeth may be set to have a very small angle relative to the feed plane so that the full, and generally excessive (relative to the feed per wiper tooth), length of the wiper tooth's cutting edge is not continuously rubbing on the machined surface that was wiped by a previous wiper tooth passing over that part of the surface.
While a face mill having wiper teeth may have them in addition to a full complement of evenly spaced primary cutting teeth, most face mills having wiper teeth replace some of the primary cutting teeth with a wiper tooth. While this is convenient and is easy to accomplish given the limited space available between successive primary cutting teeth, replacing some of the primary cutting teeth results in the primary cutting tooth that follows that replaced primary cutting tooth position to realize twice the nominal feed per primary tooth. As such it removes double the nominal amount of material, which can cause those primary cutting teeth to wear more quickly than the others.
Generally, a wiper tooth has a means of axial adjustment so that the wiper teeth can be adjusted to the desired wiper depth (relative to the furthest axially protruding primary cutting tooth) and, in the case of multiple wiper teeth, adjusted to be well aligned with all other wiper teeth. It is common, though without restriction, for there to be one wiper tooth for every three to eight primary cutting teeth.